1. Field of the Invention
The present invention relates to a magnetic recording medium, a method and an apparatus for producing a magnetic recording medium, and in particular to a high recording density magnetic recording medium preferably used for information related equipment and audio-video equipment, a method and an apparatus for producing a high recording density magnetic recording medium.
2. Description of the Related Art
Recently, high-function thin film technologies have been remarkably developed and applied in a wide variety of fields. For example, the remarkable development in the magnetic recording medium is demonstrated by the improvement in the recording density. A conventional magnetic recording medium of a so-called coating type includes a ferromagnetic material such as .gamma.-Fe.sub.2 O.sub.3 powders, CrO.sub.2 powders, or pure iron powders applied to a polymer film together with a binder such as a resin. Such a type of magnetic recording medium is used for audio and video tapes.
In order to improve the recording density, a metal thin film magnetic recording medium has been studied, which includes a ferromagnetic material such as Fe, Co, Ni or Cr vapor-deposited as a single metal material or an alloy or an insulating substrate such as a polymer film or a glass plate by ion plating, sputtering, cluster ion beam or the like. A vapor-deposition tape including such a metal material or an alloy applied to an insulating substrate by oblique vapor deposition has already been put into practice for use as a video-tape. Generally, a vapor-deposition tape is of an in-plane magnetic recording type, and includes a ferromagnetic metal film having an anisotropy (in a shape and/or in a magnetic property) in the tape plane. Such a ferromagnetic metal film is formed by obliquely growing ferromagnetic metal crystals using oblique vapor deposition.
FIG. 7 shows a conventional apparatus 70 for producing a metal thin film magnetic recording medium. The apparatus 70 utilizes a continuous winding vacuum vapor deposition to produce a metal thin film magnetic recording medium. The continuous winding vacuum vapor deposition is superior especially in the productivity and is a major candidate for a practical mass-production method.
The apparatus 70 operates in the following manner. A polymer film 72 wound around a feeding shaft 71 is continuously fed. The polymer film 72 is applied on a cooling drum 73 and wound around a winding shaft 74. Arrows A and B show the running direction of the polymer film 72. A ferromagnetic metal accommodated in a crucible 76 provided below the cooling drum 73 is irradiated with an electron beam 75 to melt and vaporize, thereby vapor-depositing the ferromagnetic metal to a surface of the polymer film 72. At this point, an unnecessary portion of the ferromagnetic metal is blocked by blocking plates 77a and 77b. In this specification, the surface of the polymer film 72 to which the ferromagnetic metal is vapor-deposited will be referred to as a "vapor-deposition surface". An area of the cooling drum 73 which is defined by the blocking plates 77a and 77b and to which the ferromagnetic metal is actually vapor-deposited on the vapor-deposition surface of the polymer film 72 will be referred to as a "vapor-deposition area". In FIG. 7, the vapor-deposition area is represented by reference numeral 72a. After the polymer film 72 having the ferromagnetic metal to a prescribed thickness is wound around the winding shaft 74, the polymer film 72 is cut or otherwise processed. Thus, a vapor-deposition tape is produced.
According to an alternative method using the apparatus 70, oxygen gas is supplied from an oxygen gas supply opening 78 to the vapor-deposition surface in the direction of arrow C (i.e., substantially opposite to the running direction of the polymer film 72), so that the depositing metal is oxidized (reactive deposition). A thin film recording medium produced by reactive deposition is formed of a ferromagnetic metal oxide or a ferromagnetic alloy oxide. In this specification, the term "thin film including a ferromagnetic metal material" refers to a thin film formed of a single ferromagnetic metal material, a thin film formed of a ferromagnetic alloy, and a thin film formed of a metal compound such as, for example, an oxide of a single ferromagnetic metal material or a ferromagnetic alloy. In the following description, the terms "ferromagnetic metal material" and "ferromagnetic metal thin film" will be used for simplicity. Unless otherwise specified, the term "ferromagnetic metal material" is replaceable with "ferromagnetic metal alloy" and "oxide of a ferromagnetic metal material", and the term "ferromagnetic metal thin film" is replaceable with "thin film including a ferromagnetic metal material"
However, the continuous winding vacuum vapor deposition has the following problems.
In order to obtain a sufficient electro-magnetic conversion characteristic with the conventional apparatus 70 shown in FIG. 7, the incident angle .alpha. of the metal depositing on the vapor-deposition surface is restricted by defining the vapor-deposition area 72a. The incident angle .alpha. is defined as being with respect to the normal to the vapor-deposition surface. The maximum incident angle .alpha. is represented as .alpha.max, and the minimum incident angle .alpha. is represented as .alpha.min.
For example, when the cooling drum 73 has a diameter of about 1 m, a vapor-deposition tape having a sufficient electro-magnetic conversion characteristic is obtained where the minimum incident angle .alpha.min. is about 40 degrees so that the component incident at a smaller angle than about 40 degrees is blocked. Since the maximum incident angle .alpha.max. cannot exceed 90 degrees, the adhering efficiency of the ferromagnetic metal reduces as the minimum incident angle .alpha. increases. The "adhering efficiency" is defined as the ratio of the weight of ferromagnetic metal deposited on the vapor-deposition surface with the total weight of the ferromagnetic metal vaporized, and is represented as a percentage.
In the case where the cooling drum 73 has a diameter of about 1 m, the minimum incident angle .alpha.min. is about 40 degrees, and the maximum incident angle .alpha.max. is about 90 degrees as shown in FIG. 7, the adhering efficiency is about 10 wt. %, which is seriously low.
As can be understood from FIG. 7, such a low adhering efficiency is due to a narrow expanding angle (vapor-deposition expanding angle) .omega. in the direction of incidence of the ferromagnetic metal. The "vapor-deposition expanding angle .omega." is defined as an angle determined by lines connecting the center of a vaporizing surface of the ferromagnetic metal in the crucible 76 and both ends of the vapor-deposition area 72a (vapor-deposition starting end and vapor-deposition terminating end) in a plane including the running direction of the polymer film 72. In the conventional method for producing a magnetic recording medium, the vapor-deposition expanding angle .omega. is restricted by the diameter of the cooling drum 73 to as small as 15 degrees.
In order to produce a ferromagnetic metal thin film having a prescribed thickness with the conventional method, the running speed of the polymer film 72 needs to be reduced due to such a low adhering efficiency. Accordingly, it is difficult to produce a vapor-deposition tape having a sufficient electro-magnetic conversion characteristic with a sufficient productivity. Under the circumstances, it has been strongly demanded to provide a method for producing a vapor-deposition tape for video equipment which has been more and more reduced in size due to a higher recording density and also a vapor-deposition tape for information equipment which has been demanded to realize higher density recording and lower cost.
In addition to the above-described problems, the present inventor has found that a ferromagnetic metal thin film produced by the conventional continuous winding vacuum vapor deposition has a problem in that the electro-magnetic conversion characteristic is lowered due to an excessively high surface roughness, which causes the space between the magnetic head and the ferromagnetic metal thin film to be excessively large. Such a problem is generally referred to as spacing loss.
FIG. 8A is a schematic of a vapor-deposition tape 80 produced by the apparatus 70 shown in FIG. 7. The polymer film 72 includes a base film 82 (for example, a PET film) and a polymer material 86 applied thereto, the polymer material 86 containing particles 84 (for example, silica particles having a diameter of about 10 nm) dispersed therein. Accordingly, the vapor-deposition surface of the polymer film 72 has a surface roughness (i.e., projections) of about 10 nm to about 30 nm. A purpose of forming a surface roughness at the vapor-deposition surface of the polymer film 72 is to lower the friction coefficient between the vapor-deposition tape and the magnetic head and thus to improve the running easiness. As shown in FIG. 8A, the ferromagnetic metal film 88 deposited on the vapor-deposition surface has projections 88a in correspondence with the projections.
The surface roughness of the ferromagnetic metal film 88 produced by the conventional method (i.e., the height of the projections 88a) is 1.5 times to twice the height of the surface roughness at the vapor-deposition surface of the polymer film 72 (which is proximate to the diameter of the particles 84). As shown in FIG. 8A, when the ferromagnetic metal film having a thickness of about 150 nm is formed on the polymer film 72, the height of the projections 88a (i.e., the surface roughness) is about 50 nm (=about 200 nm-about 150 nm), which is about 1.5 times the surface roughness of the polymer film 72 due to the silica particles 84 (about 30 nm; increase of about 20 nm). Such a large surface roughness of the ferromagnetic metal film is larger than the designed value, thus causing the problem of spacing loss. A reduction in the electro-magnetic conversion characteristic due to the excessive surface roughness brings serious problems especially in short-wavelength recording and reproduction. It is difficult to set the surface roughness of the polymer film 72 smaller because of the restriction on the usable particles. More specifically, a smaller-diameter particle has a lower dispersibility and is more expensive.
As a result of detailed studies, the present inventor has found that the increase in the surface roughness of the ferromagnetic metal film is caused by the influence of the shadowing of the component of the ferromagnetic metal deposited on the vapor-deposition surface at a large incident angle and the deposition speed of the ferromagnetic metal.
The cross section in FIG. 8A is along the running direction of the polymer film 72 during the production of the vapor-deposition tape 80. The running direction is from the right to the left of the sheet of FIG. 8A. On the vapor-deposition surface of the polymer film 72 running from the right to the left of the sheet, ferromagnetic metal comes from an upper left area of the sheet of FIG. 8A to be deposited on the polymer film 72. As shown in FIG. 7, the incident angle .alpha. of the ferromagnetic metal is high and the deposition speed is low at the deposition starting end. The incident angle .alpha. is decreased and the deposition speed is raised as the deposition terminating and is approached. As a result, the projections are developed from the forward end of the running direction and as a result, a recessed area where the ferromagnetic metal is not sufficiently supplied (i.e., shadow) is formed rear to the projections. Such a surface state of the vapor-deposition tape can be observed by an atomic force microscope (AFM). As shown in FIG. 8C, recessed areas (black areas) are formed by shadowing rear to the projections in terms of the running direction R of the tape. The recessed areas shown in FIG. 8C will be described later in the section of the comparative example 1.
FIG. 8B shows a result of observation of the ferromagnetic metal film 88 of the vapor-deposition tape 80 (FIG. 8A) by a transmission electron microscope (TEM). As shown in FIG. 8B, column-like crystals 89 of the ferromagnetic metal increase in size as the surface is approached. Such a phenomenon is considered to occur since the incident angle .alpha. and thus the deposition speed do not linearly change with respect to time due to the running of the polymer film 72 on the circumferential surface of the cylindrical cooling drum 73 according to the conventional method as shown in FIG. 7. Since the column-like crystals of the ferromagnetic metal shown in FIG. 8B are not grown in a constant direction, the vapor-deposition tape is low in anisotropy and thus is low in electro-magnetic characteristic.
A further study of the present inventor has found another problem of the conventional method. Since the vapor-deposition area is a part of the circumferential surface of the cooling drum 73, the temperature of the vapor-deposition surface curing the vapor deposition is constant, and thus the generation of the crystal nuclei and crystal growth are performed at the same temperature. As a result, the generation of the crystal nuclei is easily influenced by external disturbance, unstable and low in regularity. Accordingly, the ability of maintaining the magnetic characteristics and the square ratio in the hysterisis are reduced, which lowers the electro-magnetic conversion characteristic.